X-ray image diagnostic device, and x-ray image data correcting method

ABSTRACT

An X-ray image diagnostic device of the invention has sensitivity-information-acquisition controlling means commanding synchronization between X-ray irradiation from the X-ray source and readout of X-ray plane detector for correcting sensitivity. When sensitivity information of the X-ray plane detector is calculated using the intensities of X-rays irradiated onto the X-ray plane detector and X-rays detected by the X-ray plane detector, readout of the X-ray plane detector is performed synchronously with X-ray irradiation, and sensitivity information corresponding to variation of the amount of X-ray radiation onto the X-ray plane detector during the reading time is determined using data of every readout channel. Therefore, sensitivity information enabling an accurate sensitivity correction can be obtained from a single output image. This sensitivity information is stored in sensitivity-information-storing means and used for correcting sensitivity of X-ray image data acquired in actual measurements.

FIELD OF THE INVENTION

The present invention relates to an X-ray image diagnostic device usingan X-ray plane detector. Particularly, it relates to an X-ray imagediagnostic device having a function of correcting sensitivity to X-rayenergy of an X-ray plane detector exposed to X-rays.

RELATED ART

Conventional X-ray image diagnostic devices are constructed such that anX-ray image output from an X-ray plane detector resulting fromirradiation of X-rays onto an object is displayed on a TV monitor or thelike.

An ordinary X-ray plane detector consists of scintillators that convertX-rays transmitted through an object to light and photodiodes thatconvert the light output from the scintillators to electric charge.Detecting devices, each of which consists of a scintillator and aphotodiode, are arranged in a matrix corresponding to the individualpixels. Electric charge converted from light by each photodiode is readout via a switching device such as a thin film transistor (TFT) toproduce an X-ray image.

Sensitivity differs among the detectors due to difference in thecharacteristics of the individual scintillators and photodiodescomposing the detector. Accordingly, it is necessary to detereminesensitivity information for each detector in advance and to correctdetected signals using the sensitivity information upon imaging.

Conventionally, such sensitivity correction is carried out byirradiating X-rays of a predetermined intensity onto the X-ray planedetector with no object present to output an image from the X-ray planedetector and determining detector sensitivity information from theimage. The sensitivity information determined using the X-rays of thepredetermined intensity is also used when X-rays of a differentintensity from the predetermined intensity are irradiated onto an objectto carry out imaging. This causes a problem in that sensitivitycorrection is not accurate.

In order to solve this problem, X-rays need to be irradiated multipletimes to find the sensitivities for X-rays of various intensities. Thiscauses another problem in that the work of the operator for collectingsensitivity information increases and that the capacity of the storagemedium for storing the collected sensitivity information must beincreased.

Therefore, an object of the present invention is to provide an X-rayimage diagnostic device capable of obtaining sensitivity information forvarious X-ray intensities from a single output image and performingaccurate sensitivity correction.

DISCLOSURE OF THE INVENTION

To attain the above-mentioned object, an X-ray image diagnostic deviceof the present invention comprises an X-ray source for irradiatingX-rays onto an object to be imaged, an X-ray plane detector placed faceto face with the X-ray source for outputting X-rays transmitted throughthe object as X-ray image data, image storage means for storing theX-ray image data output from the X-ray plane detector as digital data,and sensitivity correcting means for performing sensitivity correctionof the X-ray image data stored in the image storage means, wherein saidsensitivity correcting means comprisessensitivity-information-acquisition controlling means for commandingsynchronization of irradiation of X-rays from the X-ray source andreadout of the X-ray image data from the X-ray plane detector,sensitivity-information calculating means for calculating sensitivitiesof plural readout channels of the X-ray plane detector using the X-rayimage data read out synchronously with X-ray irradiation, andsensitivity-information storing means for storing the sensitivityinformation calculated by the calculating means.

By synchronizing irradiation of X-rays from the X-ray source withreadout of X-ray image data from the X-ray plane detector andcalculating sensitivity information for each readout channel based onthe X-ray image data, sensitivity information corresponding to change inthe amount of X-ray radiation irradiated onto the X-ray plane detectorwithin the read time can be found. That is, sensitivity informationenabling accurate sensitivity correction can be obtained from a singleoutput image.

The control for synchronizing X-ray irradiation with readout of X-rayimage data performed by the sensitivity-information-acquisitioncontrolling means includes not only that for performing X-rayirradiation and readout of X-ray image data at the same time but alsothat for performing readout of X-ray image data at a desired time pointssynchronized with the X-ray irradiation.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a functional diagram showing the X-ray image diagnostic deviceof the present invention.

FIG. 2 shows a configuration of an X-ray plane detector.

FIG. 3 is a timing chart of X-ray irradiation onto the X-ray planedetector and readout of X-ray image data from the X-ray plane detector.

FIG. 4 is a view of schematically illustrating X-ray image data storedin image storage means 5 when the X-ray image data is read out from theX-ray plane detector according to the timing chart of FIG. 3.

FIG. 5 is a graph showing a luminance distribution between arbitrarilyselected channels (a) and (b) on a line of the X-ray image data.

FIG. 6 shows a timing chart of X-ray irradiation onto an X-ray planedetector 4 having a specific X-ray rise characteristic and of X-rayimage data read out from the X-ray plane detector 4.

FIG. 7 shows another exemplary timing chart of X-ray irradiation ontothe X-ray plane detector and X-ray image data read out from the X-rayplane detector.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

Embodiments of the X-ray image diagnostic device of the presentinvention will be explained with the reference to the attached drawings.

FIG. 1 is a functional diagram showing the X-ray image diagnostic deviceof the present invention.

As shown in FIG. 1, the X-ray image diagnostic device of the presentinvention has an X-ray source 3 which irradiates X-rays onto an object 2to be imaged under the control of an X-ray generator 1, and an X-rayplane detector which is placed face to face with the X-ray source 3 andoutputs transmitted X-rays through the object 2 as X-ray image data. TheX-ray image diagnostic device further comprises means for displayingX-ray image data output from the X-ray plane detector 4 as images (5-8),control means for controlling X-ray irradiation and image-signal readoutfrom the X-ray plane detector 4 (10,11,12), and an operation table 9 forinputting directions and conditions necessary for operation of thedevice etc.

The X-ray generator 1 is connected to the X-ray control means 11 andcontrols the X-ray source 3 to irradiate X-rays having a desired X-rayintensity at desired times according to control signals from the controlmeans 11.

The X-ray plane detector 4 consists of scintillators which convert theX-rays transmitted through the object to light and sensor cells whichconvert light generated by the scintillators to electric signals. Asshown in FIG. 2, a large numbers of sensor cells are arranged in amatrix. Each sensor cell constitutes one pixel. Each sensor cellcomprises a photodiode 41 which converts light output from thescintillator to electric charge, a capacitor 42 which stores theelectric charge, and a switching device 43 (thin film transistor :TFT)which reads the stored electric charge. The combination of thescintillator and photodiode can be replaced by a sensor cell of directconversion type which converts X-ray to electric charge directly.

The gate terminal of the switching device 43 is connected through acommon line to an associated line output terminal of the gate driver 45.The gate driver 45 is connected to the readout control means 12 (FIG. 1)that controls reading of signals from the X-ray plane detector 4. Thedrain terminals of the switching devices 43 in the same column areconnected in common through integration circuit to an associated inputterminal of multiplexer 46.

When X-rays are irradiated onto the X-ray plane detector 4, thephotodiodes 41 store electric charges in the capacitors 42 in proportionto the amount of radiation. The stored electric charge signals are readby operating the TFTs 43. During this operation, the multiplexer 46reads one by one in a temporal order the signals input to the inputterminal during the period of a single pulse output from each lineoutput terminal of the gate driver 45. Thus, electric charges stored forevery pixel of one line are read as an image signal. When reading of oneline is completed, reading of the next line is started. Thus, theelectric charges for every line are read out in order.

Means (5-8) for displaying X-ray image data output from the X-ray planedetector 4 comprise image storage means 5, sensitivity correcting means6, sensitivity-information storage means 7, and displaying means 8. Theimage storage means 5 stores X-ray image data output from the X-rayplane detector 4 as digital data, as mentioned above, and outputs thedigital data to the displaying device 8. The sensitivity correctingmeans 6 performs sensitivity correction of the X-ray image data storedin the image storage means 5 using sensitivity information acquired andstored in the sensitivity-information storage means 7 beforehand. Themanner of obtaining the sensitivity information will be explained later.X-ray image data corrected by the sensitivity correction is also storedin the image storage means 5. The X-ray image data stored in the imagestorage means 5 both before and after the correction are displayed onthe displaying means 8 as images.

In order to acquire the sensitivity information used in the sensitivitycorrecting means 6, the X-ray image diagnostic device according to thisembodiment further comprises sensitivity-information-acquisition controlmeans 10 which controls the X-ray control means 11 and readout controlmeans 12 so as to operate synchronously, and sensitivity-informationcalculating means 13 which calculates the sensitivity information usingimage data obtained by the X-ray plane detector 4 controlled by thesensitivity-information-acquisition control means 10. The sensitivityinformation calculated by the sensitivity-information calculating means13 is stored in the sensitivity-information storage means 7 for use incorrection processing by the sensitivity correcting means 6.

The sensitivity-information-acquisition control means 10 operates anddrives the X-ray control means 11 and readout control means 12 accordingto the operator's directions input from the operation table 9 in orderto acquire the sensitivity information. The readout control means 12operates according to directions from thesensitivity-information-acquisition control means 10 and controls theX-ray plane detector 4 so that X-ray image data is read from the X-rayplane detector 4 at a desired time synchronous with X-ray irradiation.The X-ray image data read under the control of the readout control means12 is temporally stored in the image storage means 5. Thesensitivity-information calculating means 13 reads out the X-ray imagedata for sensitivity information stored in the image storage means 5,calculates sensitivity information for each readout channel of the X-rayplane detector 4, and stores it in the sensitivity-information storagemeans 7.

Next, operation of the X-ray image diagnostic device of this embodimentwill be explained.

The procedure for acquiring sensitivity information will be explainedfirst. Acquisition of sensitivity information is started by sending asignal for starting acquisition of sensitivity information from theoperation table 9 to the sensitivity-information-acquisition controlmeans 10 when no object 2 is present.

Upon receiving the starting signal, thesensitivity-information-acquisition control means 10 sends a signal forstarting of X-ray image data readout to the readout control means 12 andat the same time sends an X-ray irradiation start signal to the X-raycontrol means 11. When the X-ray generator 1 receives the X-rayirradiation start signal, it controls the X-ray source 3 so as toirradiate X-rays of a desired intensity onto the X-ray plane detector 4.The desired intensity of the X-rays irradiated onto the X-ray planedetector 4 is an intensity capable of obtaining a sufficient amount ofsensitivity information for sensitivity correction carried out by thesensitivity correcting means 6 and may be set to a constant valuebeforehand or may be varied by sending the operator's directions throughthe operation table 9 directly to the X-ray generator 1.

FIG. 3 is a timing chart illustrating the timing of X-ray irradiationonto the X-ray plane detector 4 by the X-ray source 3 and reading ofX-ray image data from the X-ray plane detector 4.

As shown in the figure, reading of X-ray image data is started at thesame time as irradiation of X-rays onto the X-ray plane detector 4. TheX-ray image data read from the X-ray plane detector 4 by the readoutcontrol means 12 is stored in the image storage means 5. The X-ray imagedata stored in the image storage means 5 and the luminance distributionthereof are shown in FIG. 4 and FIG. 5. As illustrated, the luminancedistribution changes between arbitrarily selected readout channels (a)and (b) on the same line.

As shown in FIG. 3, since reading of X-ray image data from the X-rayplane detector 4 begins at the same time as the X-ray source 3 begins toirradiate X-rays onto the X-ray plane detector 4, a pixel at the point(a), which is detected soon after the X-ray irradiation, is not exposedto a sufficient amount of X-ray radiation. On the other hand, a pixel atthe point (b) is exposed to X-rays during the period from the beginningof the X-ray irradiation (beginning of reading) to the reading of X-rayimage data of the pixel at the point (b) As the result, the luminancedistribution between (a) and (b) comes to have the shape shown in FIG.5.

The abscissa of the graph shown in FIG. 5 corresponds to the amount ofX-ray radiation input to each pixel and therefore the sensitivities tovarious X-ray intensities between readout channels (a) and (b) can beobtained. That is, by determining the luminance distribution curvebetween (a) and (b) of the X-ray image data, it is possible to producesensitivity information of readout channels for reading image databetween (a) and (b) of the X-ray plane detector 4. In the same manner,sensitivity information for all of the readout channels of the X-rayplane detector 4 to various incident X-ray intensities can be producedby determining luminance curves of the X-ray image stored in the dataimage storage means 5 in all of the readout channel directions.

Although FIGS. 4 and 5 show that sensitivity information between twopoints on one line defined arbitrarily is read, the point (a) from whichreadout of sensitivity information is started and the point (b) wherereadout is terminated need not be on the same line but can be any pointson the X-ray plane detector 4, because readout of X-ray image data fromthe X-ray plane detector 4 is carried out for each line in order asexplained with reference to FIG. 2. Thus, sensitivity information forvarious X-ray intensities corresponding to change of the X-ray exposuretime of these detecting devices between these two points can beobtained.

The sensitivity information for each readout channel of the X-ray planedetector 4 calculated by the sensitivity information calculating means13 can be expressed by the equation S(i,k)=Q/I(i,k). Here, S representsX-ray sensitivity [C/J], Q represents electric charge density generatedin one unit-area of the photodiode, I represents intensity [J/m²] of anincident X-ray, i is the channel number on one line and k is the columnnumber. Thus, sensitivity to incident X-ray intensities I(1,1) . . .I(x,y) (x,y is the matrix size) varying between X-ray readout channelscan be calculated.

The sensitivity information for each readout channel of the X-ray planedetector 4 thus calculated by the sensitivity information calculatingmeans 13 is stored in sensitivity information storage means 7 and usedby the sensitivity correcting means 6 for sensitivity correction of theX-ray image data.

Specifically, when X-rays of a predetermined intensity is irradiatedonto an object 2 and the obtained X-ray image data is subjected to thesensitivity correction using the sensitivity S for the intensity of theirradiated X-rays, the detected X-ray intensity D′ is correctedaccording to the following equation:D″=D′×S

D′: Electric charge density generated in one unit-area of photodiode[C/m²]

D″: Corrected X-ray intensity [J/m²]

S: X-ray sensitivity [C/J]

If no sensitivity information is available for the actual intensity ofthe irradiated X-rays, it is possible to use a value interpolated fromtwo points of sensitivity information for larger and smaller X-rayintensities.

While sensitivity information for all channels from the beginning to theend of the reading of the X-ray image data is calculated in thisembodiment, sensitivity information for a plurality, but not all of thechannels, can be calculated and sensitivity information between thesechannels be found by interpolation.

According to this embodiment, sensitivity information for various X-rayintensities can be obtained by starting X-ray image data readout fromthe X-ray plane detector 4 at the same time as irradiating X-rays ontothe X-ray plane detector 4 and determining sensitivity information fromthe detected X-ray intensity for each pixel.

The above embodiment was explained with regard to the case where thereadout of X-ray image data from the X-ray plane detector 4 is startedat the same time as irradiating X-rays onto the X-ray plane detector 4.However, as known in the art, it takes a certain amount of time from thestart of X-ray irradiation to achieve a desired X-ray intensity. Acontrol considering this X-ray rise characteristic is necessary.Hereinafter, a method of acquiring sensitivity information for eachreadout channel of the X-ray plane detector 4 taking the risecharacteristic into account will be explained taking as an example anX-ray plane detector 4 having an image-readout number of 30 images persecond.

FIG. 6 is a timing chart of X-ray irradiation onto the X-ray planedetector 4 having an X-ray rise characteristic and reading of X-rayimage data from the X-ray plane detector 4.

When irradiation of X-rays onto the X-ray plane detector 4 is started,the sensitivity-information calculating means 13 obtains X-raywaveform-information from the X-ray generator 1 and calculates the risetime of the X-rays (T_(s) in FIG. 6) using the waveform-information. Thesensitivity-information calculating means 13 determines image data,which is read from the X-ray plane detector 4 within the rise time(T_(s)) of the X-ray, among X-ray image data stored in the image storagemeans 5. Specifically, when the number of image readouts from the X-rayplane detector 4 is 30 images per second, the time required to read oneimage is 33.3 ms. Defining the blanking time, i.e., the time within thisperiod when only X-ray irradiation is carried out and X-ray image datais not read, as T_(b), the time Tr required solely for reading X-rayimage data becomes:T _(r=)33.3−T _(b)  (1)

If the number of pixels of X-ray image data read from the X-ray planedetector 4 is (x×y), the time T_(p) required for reading one pixelbecomes:T _(p)=(33.3−T _(b))/(x×y)  (2)

Pixels which are read within a time satisfying the following conditionfrom the beginning of reading of the X-ray image data are pixels whichare read within the X-ray rise time (T_(s)).T _(s) ≧T _(p) ×n(n=0,1,2 . . . )  (3)

The sensitivity-information calculating means 13 calculates sensitivityinformation for each readout channel of the X-ray plane detector 4 fromX-ray image data read by the X-ray plane detector 4 after the timesatisfying the above equation and stores it in the sensitivityinformation storage means 7. Image data within the X-ray rise time whichwas not obtained, that is, sensitivity information within the time, canbe found using the sensitivity information found from image data afterthe rise time and stored in the sensitivity information storage means 7,by extrapolation using for example a multinomial approximation method.

While the case where the X-ray waveform has a rise characteristic wasexplained in the embodiment shown in FIG. 6, the X-ray waveformgenerated by the X-ray generator 1 is generally known to have not only arise characteristic but also a fall characteristic. For dealing withsuch a characteristic, the same process as used for the risecharacteristic can be applied to obtain sensitivity information for eachreadout channel of the X-ray plane detector 4 while avoiding theinfluence of the fall characteristic.

Although X-ray image data detected during the rise time or fall time isnot used for calculating sensitivity information in the aboveembodiment, a delay time corresponding to the rise time may be set forthe sensitivity-information-acquisition control means 10 to control thereadout control means 12 and the X-ray control means 11.

Such an embodiment is shown in FIG. 7. FIG. 7 is a timing chart of X-rayirradiation onto the X-ray plane detector and of X-ray image datareadout from the X-ray plane detector.

In this embodiment, as illustrated, reading of X-ray image data from theX-ray plane detector 4 is started at a predetermined time after thestart of X-ray irradiation and terminated a predetermined time beforethe termination of X-ray irradiation. In this embodiment, X-ray imagedata is not read during the X-ray rise time and fall time. The timescorresponding to the rise and fall of the X-ray waveform can be setusing the X-ray waveform obtained from the X-ray generator 1 as in theembodiment shown in FIG. 6

The method of synchronizing the reading of X-ray image data with theirradiation of X-rays is not limited to that of the embodiments shown inFIGS. 3 and 6 but the delay time between X-ray irradiation and X-rayimage data reading or the reading time can be set arbitrary. In thiscase, the luminance distribution (sensitivity information) of anarbitrary range of the X-ray intensity can be obtained.

As explained above, according to the X-ray image diagnostic device ofpresent invention, sensitivity information for performing accuratesensitivity correction can be obtained from a single output image. Theoperator's work for collecting sensitivity information can therefore bereduced and increase in the capacity of a storage medium for storing thesensitivity information can be minimized.

1. An X-ray image diagnostic device comprising an X-ray source forirradiating X-rays onto an object to be examined, an X-ray planedetector placed opposite to the X-ray source for outputting X-ray imagedata corresponding to X-rays transmitted through the object, imagestorage means for storing the X-ray image data output from the X-rayplane detector as digital data, and sensitivity correcting means forperforming sensitivity correction on the X-ray image data stored in theimage storage means, wherein said sensitivity correcting means comprisessensitivity-information-acquisition controlling means for commandingthat X-ray irradiation from said X-ray source be synchronized withreadout of the X-ray image data from the X-ray plane detector,sensitivity-information calculating means for calculating sensitivityinformation for a plurality of readout channels of the X-ray planedetector using the X-ray image data read out synchronously with X-rayirradiation, and sensitivity-information storage means for storing thesensitivity information calculated by the calculating means.
 2. TheX-ray image diagnostic device of claim 1, wherein saidsensitivity-information calculating means calculates the sensitivityinformation using X-ray image data other than data detected during therise time and fall time of irradiated X-rays.
 3. The X-ray imagediagnostic device of claim 1, wherein saidsensitivity-information-acquisition controlling means controls the X-rayplane detector such that the X-ray image data are read out at apredetermined time synchronously with X-ray irradiation.
 4. The X-rayimage diagnostic device of claim 1, wherein saidsensitivity-information-acquisition controlling means sets a delay timecorresponding to the rise time of the irradiated X-rays for reading outthe X-ray image data.
 5. The X-ray image diagnostic device of claim 1,wherein said sensitivity correcting means performs sensitivitycorrection using a value obtained by interpolating from sensitivityinformation for at least two points of a plurality of readout channelscalculated by said sensitivity information calculating means.
 6. TheX-ray image diagnostic device of claim 1, wherein saidsensitivity-information-acquisition controlling means determines X-rayimage data readout timing based on X-ray waveform information obtainedfrom the X-ray source.
 7. A method of correcting X-ray image datatransmitted through an object to be examined and detected by an X-rayplane detector by calculating sensitivity information for readoutchannels of the X-ray plane detector using the intensities of irradiatedX-rays and of detected X-rays and using the sensitivity information,wherein readout of the X-ray plane detector is performed synchronouslywith X-ray irradiation and sensitivity information corresponding tovariation of the amount of X-ray radiation on the X-ray plane detectorduring the readout time is determined.
 8. An X-ray image diagnosticdevice comprising an X-ray source for irradiating X-rays onto an objectto be examined, an X-ray plane detector placed opposite to the X-raysource for outputting X-rays transmitted through the object as X-rayimage data, image storage means for storing X-ray image data output fromthe X-ray plane detector as digital data, sensitivity correcting meansfor correcting sensitivity of the X-ray image data stored in the imagestorage means and for storing sensitivity corrected X-ray image data inthe image storage means again, sensitivity-information-acquisitioncontrolling means for commanding that X-ray irradiation from the X-raysource be synchronized with readout of X-ray image data from the X-rayplane detector, sensitivity-information calculating means forcalculating sensitivity information for each readout channel of theX-ray plane detector out of the readout X-ray image data,sensitivity-information storing means for storing sensitivityinformation calculated by said calculating means for correctingsensitivity of the X-ray image data by said sensitivity correctingmeans.